1Then the Pharisees and Sadducees came, and testing Him asked that He would show them a sign from heaven. 2He answered and said to them, "When it is evening you say, "It will be fair weather, for the sky is red'; 3and in the morning, "It will be foul weather today, for the sky is red and threatening.' Hypocrites! You know how to discern the face of the sky, but you cannot discern the signs of the times. 4A wicked and adulterous generation seeks after a sign, and no sign shall be given to it except the sign of the prophet Jonah." And He left them and departed.
Once again the Pharisees and Sadducees are asking for a sign from heaven. In Matthew 12:38-42, some scribes and Pharisees asked Jesus for a sign from Him. Again He called them an evil and adulterous generation. Again, Jesus said the only sign they would be given is the sign of the prophet Jonah.
According to my dictionary, discern means to perceive something obscure or concealed. They were able to perceive the signs of the weather, but were not able to perceive the signs of the times. The signs were there. They may have been obscure or concealed, but the signs of the times were there. The only blatant sign that would be given them would be the sign of the prophet Jonah.
What did Jesus mean by the signs of the times? They wanted a sign from heaven. But the kingdom of heaven is upon them. I believe that the signs of the times meant the signs that the kingdom of heaven is here. Jesus came to fulfill the prophesy, to fulfill the law, to bring salvation. These are the signs that were there, signs of the times. But they didn't perceive them. Instead they were asking for a sign from heaven.
But there would be one sign given them, the sign of the prophet Jonah. And what is that sign? Jesus explained it in Matthew 12:38-34. Just as Jonah spent three days and three nights in the belly of a fish, Jesus will spend three days and three nights in the heart of the earth. The sign will be His resurrection.